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294: OPERATIVE DENTISTRY
A High-Fusing Porcelain May Be Made Low-Fusing by repeated
fusing and grinding.
In Building a Filling by Layers the first layer should be fused
to a state of high biscuit otherwise the process of fusing the sub-
sequent layers will over-fuse the first.
High Biscuit Fuse. Heating the porcelain sufficient to obtain
shrinkage, but not enough to glaze.
Fine Grinding. The more finely porcelain is ground the lower
the fusing point from the same formula and the greater the shrink-
age.
Size of Mass. The larger the mass the greater the length of
time required to fuse.
Amount of Flux. The more flux a porcelain contains the great-
er the liability to bubble, which liability increases as the tempera-
ture is raised.
Shrinkage in Fusing. High fusing porcelains shrink from fif-
teen to twenty-five per cent. Low fusing porcelain shrinks from
twenty to thirty-five per cent.
Spheroiding. All porcelains have a great tendency to spheroid
Avhen over-fused.
A Basal Body is porcelain composed of basal ingredients and the
l>igments.
A Foundation Body is one composed of basal ingredients to which
has been added a flux to increase fusibility, and has been ground
less fine than enamel body to raise fusing point and give stability
as to form.
An Enamel Body is a basal body which has been more finely
ground and to which there has been added more flux to increase
fusibility.
The Advantages of the Porcelain Inlay. When skillfully made
they more nearly harmonize with tooth structure in appearance.
Thermal changes do not readily affect the pulp in vital cases as
porcelain is not as good a conductor as metal.
Margins of cavities Avell filled with porcelain are not readily at-
tacked by caries, as cement dissolves out of the margin to a depth
only equal to the breadth of the line exposed. Patients are relieved
of sitting Avith the rubber dam in position for protracted periods.
The Disadvantages of the Porcelain Inlay. The friability of
porcelain restricts its use to locations removed from great stress.
It is necessary to omit the marginal bevel in all cavities, as the